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Your aunt, soon to be feverish and aching, double-dipped her potato chip at the family's holiday bash. Your son's sick classmate came to preschool anyway and sneezed into his hands, then wiped them on the craft table. Your co-worker, certain of his indispensability to the office, is shivering and coughing at the desk next to yours.

A few days later, like an increasing number of local residents, you're offering the world your own party platter of influenza virus, accompanied by the usual misery-inducing symptoms: sore throat, headache, fatigue, fever over 100 degrees, coughing and aching muscles and joints.

In southwestern Pennsylvania, flu season hit earlier and harder than usual and now is in full swing, with 726 Allegheny County residents testing positive for influenza virus as of Dec. 29, according to Allegheny County Health Department Interim Director Ronald Voorhees. Two residents, both over the age of 65, have died, he said.

But those results, Dr. Voorhees said, represent a fraction of the true number of influenza-infected area residents, only a small number of whom will seek medical care and even fewer of whom will be tested.

"We know there's a lot more than that," Dr. Voorhees said of the official number of flu-infected Allegheny County residents. "The actual number is much larger, and it's climbing."

The Pennsylvania Department of Health said a total of 7,181 state residents had tested positive for influenza by the end of December, with nearly 3,200 flu cases reported in just the final week of the year. That number is far higher than at this point in recent years; in the 2010-11 season, for instance, the state had reported just 265 cases by the end of December, and saw a total of 9,928 during the entire season.

The Centers for Disease Control and Prevention reported that 41 states had widespread flu outbreaks by the end of December, up from 31 states the previous week, and more than 2,257 people have been hospitalized with flu-like symptoms.

By another measure, Dr. Voorhees said, the percentage of people complaining of flu-like symptoms at six Pittsburgh hospitals that gather and report such information has increased from the usual 2.5 percent of patients to about 6 percent. That number, Dr. Voorhees said, typically signals a flu season has reached full strength.

Even so, local doctors urged flu shots for anyone who hasn't already received one this season, with several of them pointing out that the virus causing most of the sickness now -- influenza Type A -- might give way later in the season to another strain that is circulating but not predominant at the moment.

About 90 percent of current flu cases are a result of the Type A strain, while most of the remainder is caused by the Type B strain. This season's vaccine is an almost-perfect match for the Type A virus, although only about two-thirds of a match for the Type B strain, Dr. Voorhees said.

Last year, the flu season did not start until the end of February, and peaked weeks later, said Thomas Campbell, who heads emergency medicine for the West Penn Allegheny Health System. Flu season can start any time after Thanksgiving and can last into April, experts said.

As a result, it's especially important, they said, to annually immunize children between 6 months and 2 years old, pregnant women, people older than 65 and anyone with an underlying health problem such as heart disease, lung disease, diabetes or a compromised immune system.

Health care workers and students are also high-priority groups to get a yearly flu shot, which provides immunity for about six months, he said.

"This is a year [that], even if you're not in those categories, it would be a good idea to get the vaccine," Dr. Campbell said. "You wouldn't be wasting your time, money and effort to get the vaccine."

Contrary to the claims of some people, the flu vaccine contains only protein fragments of the virus, not the whole virus needed to cause illness, and cannot itself cause the flu, experts said. The body's immune response to the virus proteins, however, can cause a slight fever and some muscle aches as the immune system builds resistance.

Resistance begins to build within three to five days after receiving the immunization, and reaches full strength in about two weeks. Health experts said getting enough sleep -- seven to nine hours per night for adults -- eating well and drinking lots of liquids also can help the body fight the flu and keep it from turning into something more serious, such as pneumonia. It also can be a good idea to avoid enclosed, crowded places during flu season; wash hands after returning from public places and before eating or drinking; and limit unnecessary social contact, Dr. Campbell said.

"It sounds a little antisocial but I would wave instead of shaking hands, or pat someone on the back, especially this time of year," he said.

For the families of babies under 6 months old and therefore too young to receive the vaccine, it's important to "cocoon" the baby by immunizing everyone around the baby, especially young children who often bring home germs from day care and school and share them generously, said Marian Michaels, a pediatric infectious disease specialist at the University of Pittsburgh and Children's Hospital of Pittsburgh of UPMC.

The emergency room at Children's has seen more teenagers suffering from flu -- including very critical illness -- this year than in the past, Dr. Michaels said. And some of those sick children might have stayed healthy, or at least might not have gotten as sick, if they had gotten a flu shot, she said.

"It breaks my heart," Dr. Michaels said. "The majority of people who are very critically ill haven't been vaccinated."